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Tongue worms
Linguatula taenioides, a pentastomid parasite of sheep
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Subphylum: Crustacea Class: Maxillopoda Subclass: Pentastomida Diesing, 1836
Orders
Cephalobaenida Porocephalida

The Pentastomida are a group of parasitic invertebrates commonly known as tongue worms due to the resemblance of some species to a vertebrate tongue.

There are about 100 extant species of pentastomids; all are obligate parasites with correspondingly degenerate anatomy. Adult tongue worms vary from about 1 cm to 14 cm in length, and parasitize the respiratory tracts of vertebrates ^[1]. They have five anterior appendages. One is the mouth; the others are two pairs of hooks which they use to attach to the host. This arrangement led to their scientific name, meaning "five openings", but although the appendages are similar in some species, only one is a mouth.

Their affinity with other invertebrates is uncertain. They grow by moulting, which suggests they belong to the Ecdysozoa. Historically, they were considered to deserve a phylum of their own, but in 1972, Wingstrand showed similarities in the structure of spermatozoa between pentastomids and branchiurans (fish lice), a group of parasitic crustaceans ^[2], and this relationship is supported by molecular evidence ^[3].

However, fossils from the Upper Cambrian identified as pentastomids (Heymonsicambria, Haffnericambria, Bockelericambria) suggest that the pentastomids branched very early and may be an outgroup to the other arthropods ^[4].

Alternative names for the Pentastomida include Pentastoma (strictly a genus name), Linguatulida, and Acanthotheca.

This article follows Martin and Davis in placing Pentastomida in the class Maxillopoda within the subphylum Crustacea ^[5]. Martin and Davis present this classification of the pentastomids down to the level of families:

Subclass Pentastomida Diesing, 1836

• Order Cephalobaenida Heymons, 1935
  • Cephalobaenidae Fain, 1961
  • Reighardiidae Heymons, 1935
• Order Porocephalida Heymons, 1935
  • Armilliferidae Fain, 1961
  • Diesingidae Fain, 1961
  • Linguatulidae Heymons, 1935
  • Porocephalidae Fain, 1961
  • Sambonidae Fain, 1961
  • Sebekiidae Fain, 1961
  • Subtriquetridae Fain, 1961

Pentastomids live in the upper respiratory tract of reptiles, birds, and mammals, where they lay eggs. The eggs are either coughed out by the host or leave the host body through the digestive system. The eggs are then ingested by an intermediate host. The first larva hatches in the intermediate host and breaks through the wall of the intestine. It then forms a cyst in the intermediate host's body. The pentastomid reaches the main host when the intermediate host is eaten by the main host.

• Linguatula serrata is a parasite of dogs. The adult lives in dogs' respiratory tracts and noses. The eggs leave the host body through the dog's nasal mucus and will be ingested by small herbivores. Dogs will then be infected by free-living larvae or through the intestines of infected prey. Linguatula may also infect humans.

1. ^ Cooper Curtice (1890). The animal parasites of sheep. Washington: Government Printing Office. 
2. ^ K. G. Wingstrand (1972). Comparative spermatology of a pentastomid, Raillietiella hemidactyli, and a branchiuran crustacean, Argulus foliaceus, with a discussion of pentastomid relationships. Kong. Danske Videnskabelige Selsk. Biol. Skrift. 19: 1–72. 
3. ^ L. Abele, W. Kim, and B. E. Felgenhauer (1989). Molecular evidence for inclusion of the phylum Pentastomida in the Crustacea. Mol. Biol. Evol. 6: 685–691. 
4. ^ D. Walossek and K. J. Müller (1998). "Cambrian "Orsten"-type arthropods and the phylogeny of Crustacea", in R. A. Fortey and R. H. Thomas: Arthropod Relationships. London: Chapman & Hall, 139–143. 
5. ^ Joel W. Martin and George E. Davis (2001). An Updated Classification of the Recent Crustacea. Natural History Museum of Los Angeles County.